Heterochrony explains convergent testis evolution in primates
The testis displays striking anatomical divergence among primates. Multi-male species, such as chimpanzees, have recurrently evolved large testicles relative to single-male species, such as humans. However, the developmental mechanisms behind testis divergence and whether they involve convergent molecular changes, have remained unknown. Through comparative analysis of transcriptomic data, we show that a species’ testis expression profile, like testis size, can be a reliable indicator of mating type among primates, and possibly murids. Differential expression, in turn, largely reflects changes in the relative proportions of somatic/pre-meiotic versus meiotic/post-meiotic cell types. By studying mouse and macaque testis development, we find that single-male species’ testis expression profiles are paedomorphic relative to multi-male species’ profiles. For instance, human and gorilla testis profiles resemble those of adolescent mice. Our results suggest that heterochronic shifts involving conserved transcription regulators have been repeatedly employed in primate evolution, leading to rapid, convergent changes in testis size and histology.